Sleep

Sleep consists of NREM (N1, N2, N3) and REM stages cycling every ~90 minutes. N1 is light transitional sleep. N2 is the most abundant stage; sleep spindles (12–15 Hz bursts from thalamocortical circuits) are associated with motor learning and memory consolidation, and K-complexes suppress arousal. N3 (slow-wave sleep / SWS) features delta waves and is the most restorative stage — dominated by deep hippocampal-cortical dialogue for declarative memory consolidation. REM sleep is characterised by cortical activation, rapid eye movements, and muscle atonia; it is associated with emotional memory, procedural learning, creative insight, and the "offline therapy" function of emotionally processing aversive experiences. SWS dominates early in the night; REM is longer and more intense in later cycles.

Borbély's (1982) two-process model proposes that sleep timing is determined by two interacting processes. Process S (sleep homeostasis): adenosine released during wakefulness accumulates in the basal forebrain and signals "sleep need" — it rises continuously during waking (producing progressive sleepiness) and falls during sleep (as adenosine is cleared). Caffeine works by blocking adenosine A1 and A2A receptors. Process C (circadian drive): the SCN generates a roughly 24-hour oscillation in alertness that is roughly anti-phasic to sleep pressure during the day — the "wake-maintenance zone" in the early evening occurs because the circadian alerting signal is at its peak precisely when sleep pressure is also high, keeping us awake until a preferred bedtime. Sleep onset occurs when the circadian alerting signal drops and the accumulated sleep pressure tips the balance.

Different stages of sleep support different types of memory. SWS (slow-wave sleep) consolidates declarative memories (episodic — personal experiences; semantic — factual knowledge). The dominant mechanism is hippocampal-cortical dialogue: during SWS, the hippocampus replays recently encoded memory traces (hippocampal sharp-wave ripples occur synchronously with cortical slow oscillations and thalamic spindles), gradually transferring them from hippocampal temporary storage to distributed cortical long-term storage. REM sleep is associated with procedural and implicit memory consolidation, emotional memory processing, and synaptic downscaling. Studies cutting specifically into SWS or REM impair the corresponding memory types. Total sleep deprivation produces broad memory consolidation failure — something no amount of subsequent rest fully compensates.

Insomnia (difficulty initiating or maintaining sleep, or non-restorative sleep, with daytime impairment) is the most prevalent — affecting ~10–15% chronically. CBT for insomnia (CBT-I) is the first-line evidence-based treatment, more effective and more durable than hypnotic medication. Narcolepsy is caused by loss of orexin/hypocretin-producing neurons in the lateral hypothalamus (typically through autoimmune destruction), producing excessive daytime sleepiness, cataplexy (sudden loss of muscle tone triggered by emotion), sleep paralysis, and hypnagogic hallucinations. Obstructive sleep apnoea (OSA) involves repeated upper airway collapse during sleep, producing oxygen desaturation and micro-arousals; treated with CPAP. Delayed sleep phase disorder reflects a circadian phase delay (late chronotype extremes); treated with bright morning light, chronotherapy, and low-dose evening melatonin.